The present invention relates to a positive-working radiation-sensitive coating solution which is particularly suitable for preparing photoresist layers.
Positive-working photoresist solutions are known which are based on naphthoquinone diazides or on a combination of compounds which can be split by acid and compounds which, on exposure to actinic radiation, form a strong acid, respectively. Such photoresist solutions are widely used in the production of printed circuits and microelectronic components, and for chemical milling and the like.
In these applications, the photosensitive coating solutions are normally supplied by manufacturers, and the users then apply the solutions to an appropriate layer support, for example, silicon wafers or copper-laminated plates of an insulating material, whereafter the layers are dried. Such single coating methods are more difficult to perform and more susceptible to trouble than the industrial coating of endless supports in continuous processes, which is customary in the production of presensitized printing plates or dry resist films. When individual supports are coated by the user, it is always more difficult to maintain exactly reproducible standard coating conditions.
The problems arising in this context are caused not only by the type and amount of the solid layer constituents but also to a considerable extent by the type of solvent (usually organic) employed. Since many users, who only coat small numbers of supports, do not possess installations for a recovery or environmentally safe disposal of the solvent vapors, only solvents which are not injurious to health are permitted. As standards governing the use of organic solvents in laboratories have become considerably more strict in recent times, the choice of solvents has become severely limited.
Nevertheless, most of the commercially available photoresist solutions used in practice contain a solvent mixture. This is true because a suitable photoresist solution must dry to give a layer which is as uniform as possible and free from streaks, even when the temperatures and drying conditions are not identical in all cases. Attempts to achieve this objective have usually involved providing a solvent mixture comprised of several components, each having a different solvency characteristic relative to the layer constituents. Each component of the typical solvent mixture also possesses a different evaporation number, so that a gradual solidification of the layer takes place in the course of the drying process and inhomgeneities due to a premature separation of individual constituents are thereby avoided.
U.S. Pat. No. 3,634,082 teaches that ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, aliphatic esters (e.g., butyl acetate), aliphatic ketones (e.g., methyl-isobutyl ketone and acetone), dioxane, xylene and halogenated aromatic compounds (e.g., chlorinated xylene, benzene and toluene) are suitable solvents for positive-working photoresist solutions based on 1,2-quinone diazides. Essentially the same solvents are employed for positive-working photoresist mixtures based on compounds which can be split by acid, as described, for example, in U.S. Pat. No. 4,101,323 and in European Patent Application No. 42,562.
In general, the main constituents of technical-grade photoresist solutions are ethylene glycol derivatives, such as monomethyl and monoethyl ethers, the corresponding ethers of diethylene glycol or ethylene glycol ethyl ether acetate. These solvents display good solvent powers for all customary layer components, favorable boiling points and evaporation numbers, and reasonable prices. However, the layers obtained using these compositions are not in all cases faultless. Moreover, the permissible values for their concentration in atmosphere of industrial laboratories have recently been reduced.